Those muzzies are heavy for sure. I cut one of my F&D seats shorter. It is all metal and a simple design without little parts to break. You can take about an inch off the back.
The threaded rod screwed into the aluminum round is fixed with superglue. Make a note of the length it sticks out from the seat.
Get a replacement stainless or grade 8 bolt with 2" or more of threads and a nut to go with it. I want to say the thread is 5/16-24 NF. With a vise and some care you might be able to back that threaded rod out without damaging it, but with a new bolt in hand you don't have to re-use the original and can use the bolt head to turn it in.
Now you can cut down the aluminum round off to the length you want. The rear foot holder is pinned through with a roll pin, so there is a limit to how much you can shorten it. An original F&D measures 1-3/4" behind roll pin. Should be able to cut off 1" and leave 3/4" for threads.
I don't know if the centered hole is already drilled deep enough, or if you will need to make the hole a little deeper to get the bolt in. Just don't drill the roll pin. Either way you should not need to tap threads since the steel threads of the new bolt will make their own threads in the aluminum as long as the drill hole is not too small.
When you are satisfied with depth, back the bolt out, put the nut on the bolt down to the head, then put superglue or loctite in on the tip of bolt and run it back into the seat. Then move the nut down toward the seat and cut off the bolt to the same length as the original threaded rod. Clean up/chamfer the cut, then use that nut to clean the threads while removing it.